TI 83 Plus Assembly/Advanced

Advanced z80 programming techniques

Optimizations

xor a

A common shortcut to set the accumulator (A) to zero is by xor'ing it against itself:

; instead of doing this
  ld a,0
; you can do this
  xor a

This is incredibly widespread and won't affect the readability of your code. In fact, it is so common that it might even be more readable than using "ld a,0". But be warned that "xor a" modifies the flags (it sets the z flag and resets the carry, for example) whereas "ld a,0" doesn't modify any flags.

ld (addr),reg16

When writing to two consecutive bytes in memory, it can often be faster and smaller to use a 16-bit register rather than two loads with a.

; rather than
  ld a,$10
  ld (penCol),a
  ld a,$20
  ld (penCol),a
; you can do
  ld de,$2010
  ld (penCol),de

This loads e into (penCol), and d into (penCol+1). penCol+1 is penRow. Keep in mind, however, that using hl to read/write from addresses in memory is (4 t-states) faster and one byte smaller than using de/bc.

X <= A <= Y

If you want to know if the value of the accumulator is between two other values, eg. 10 and 25, you might be tempted to do something like this:

  cp 10      ; check if a >= 10
   jr c,wrongValue
  cp 26      ; check if a <= 25
   jr nc,wrongValue
; do whatever you wanted to do
wrongValue:
; ...

However, rather than doing two comparisons and jumps, you can change the first cp into a subtraction. This turns any number less than 10 into a negative number. For example:

  sub 10      ; a value of 10 now equals 0
  cp 26-10    ; because of the sub 10, we now need to check if a is between 0 and 16, not 10 and 26
   jr nc,wrongValue
; do whatever you wanted to do
wrongValue:
; ...

This method lets you avoid the first jr.